Wikipedia:Reference desk/Archives/Science/2009 December 7

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December 7

Change in kinetic energy

It's pretty easy to show, classically, that the observed change in kinetic energy doesn't depend on the frame of reference of the observer: it is a direct result from the equation relating the kinetic energy in a certain reference frame with that of the center of mass reference frame. How can it be shown that the change in kinetic energy (or energy), relativistically, doesn't depend on the reference frame? Do you have to go into the mathematical details to derive this result, or is there an a priori way of coming to the same conclusion?

A second, related question: Does an object's potential energy change between reference frames? I would think it would, because an object's potential energy depends on the relative distance between two objects, which, by Lorentz contraction, changes with reference frame. —Preceding unsigned comment added by 173.179.59.66 (talk) 00:18, 7 December 2009 (UTC)

That what you said is not even true classically, let alone relativistically. Dauto (talk) 03:12, 7 December 2009 (UTC)

It was (is) difficult to understand your question. That's probably why you got no answers. Also I don't think this field is well studied. Dauto: what is not true classically? Ariel. (talk) 09:16, 7 December 2009 (UTC)

The OP said "It's pretty easy to show, classically, that the observed change in kinetic energy doesn't depend on the frame of reference". Well, that's not true. the change in kinetic energy DOES depend on the frame of reference. Dauto (talk) 14:46, 7 December 2009 (UTC)

Why not, assuming that the system is closed? If the total kinetic energy in a certain reference frame is K, and the total kinetic energy in the center of mass reference frame is K_0, and the velocity of the center of mass relative to the reference frame in question is V, then K = K_0 + MV^2/2 (where M is the total mass). So ΔK = ΔK_0 (V won't change if it's a closed system). So the change in total kinetic energy will always be the same as the change in total kinetic energy of the center of mass, and thus the change in kinetic energy will always be the same. —Preceding unsigned comment added by 173.179.59.66 (talk) 17:45, 7 December 2009 (UTC)

Why should we assume the system is closed? Dauto (talk) 18:37, 7 December 2009 (UTC)

You aren't making sense. The kinetic energy in the centre of mass frame is zero. Or are you talking about the centre of mass of an n-body (n>1) system? It is easier to consider one object: If in my frame a 2kg object is moving at 1 m/s and speeds up to 2 m/s its KE increases from 1J to 4J. If your frame is moving at 1 m/s relative to mine in the same direction as the object then in your frame it starts off at rest (0J) and speeds up to 1 m/s (1J). In my frame the increase in energy was 3J, in yours it was 1J. As you can see, the change it kinetic energy is dependent on the frame of reference. (That's the classical view, the relativistic view is similar and gets the same overall conclusion.) --Tango (talk) 19:11, 7 December 2009 (UTC)

I see what he's saying. It's true if momentum is conserved. That said, energy isn't conserved, so it's not a closed system, and it would seem pointless to assume momentum is conserved. I guess it's useful when you're talking about energy changing between kinetic and other forms. For example, if there are two balls each with a mass of 1 kg moving towards each other at 1 m/s that stick when they hit, the amount of energy lost in the collision is 2J regardless of reference frame. 67.182.169.172 (talk) 01:46, 8 December 2009 (UTC)

Yes, thank you...but how do you show this?

Potential energy certainly changes. Consider two identical springs, one held highly compressed by a (massless) band. Now zip past them at relativistic speed; the total energy of each must scale by γ, and you must see some of that increase in the compressed spring as additional potential energy because the other one has the same rest mass, thermal energy, and (rest-mass-derived) kinetic energy and the difference in energy is larger the compression energy in the springs' rest frame. --Tardis (talk) 15:38, 7 December 2009 (UTC)

Okay, it appears that I'm bad at making sense. 1) I wanted to say at the beginning that the system was closed, like in a collision, I just forgot to mention it. 2)It is a many body system (ie, as in a collision)...so basically, I'm asking if the change in kinetic energy in a collision (elastic or inelastic) is the same in all reference frames. —Preceding unsigned comment added by 173.179.59.66 (talk) 01:08, 8 December 2009 (UTC)

Lets say you have a system of $n\,$ particles with masses $m_{i}\,$, $i=1,2,\dots ,n\,$. The kinetic energy is given by $K=E-\sum _{i}^{n}m_{i}c^{2}\,$ where $E\,$ is the total energy. Now suppose that those particles suffer a series of collisions (which may be elastic or not) and after the collisions there are ${\bar {n}}\,$ particles with masses ${\bar {m}}_{i}\,$, $i=1,2,\dots ,{\bar {n}}\,$. The kinetic energy is given by ${\bar {K}}={\bar {E}}-\sum _{i}^{\bar {n}}{\bar {m}}_{i}c^{2}\,$ where ${\bar {E}}\,$ is the total energy after the collisions. The change in kinetic energy is ${\bar {K}}-K=({\bar {E}}-\sum _{i}^{\bar {n}}{\bar {m}}_{i}c^{2})-(E-\sum _{i}^{n}m_{i}c^{2})=\sum _{i}^{n}m_{i}c^{2}-\sum _{i}^{\bar {n}}{\bar {m}}_{i}c^{2}\,$, where the $(E-{\bar {E}})\,$ term cancles since the system is isolated and energy is conserved. Note how the final result depends only on the rest masses which are independent of the referencial used. I hope that helps a bit. Dauto (talk) 22:22, 8 December 2009 (UTC)

Redox rxn. How can I tell if a reaction is redox or not?

How can I tell if a reaction is a redox reaction just by looking at the chemical equation? Can someone show me an example? Thank you.161.165.196.84 (talk) 04:31, 7 December 2009 (UTC)

A redox reaction is one where the oxidation numbers of some of the elements is changing in the reaction. All you do is assign oxidation numbers to every element in the chemical reaction. If the oxidation number for some of the elements is different on the left from on the right, then it is a redox reaction. If all of the oxidation numbers stay the same on both sides, then it is not a redox reaction. But you need to actually know how to assign oxidation numbers before you can do anything else here. Do you need help with that as well? --Jayron32 04:34, 7 December 2009 (UTC)

Yes, that would be great. My understanding is this: Hydrogen is usually +1, Oxygen is usually -2. In binary ionic compounds the charges are based on the cation's (metal) and the anion's (non-metal) group in the Periodic Table. Polyatomic ions keep their charge (Ex// Phosphate is -3, Nitrate is -1).

Now, my textbook says the following and I am not sure what this means: "In binary molecular componds (non-metal to non-metal), the more "metalic" element tends to lose, and the less "metalic" tends to gain electrons. The sum of the oxidation number of all atoms in a compound is zero." I'm not quite sure what the first part affects, but the second part is simply saying that once all oxidation numbers have been assigned, the sum of those numbers should be zero. Is this correct? —Preceding unsigned comment added by 161.165.196.84 (talk • contribs)

Yeah, that's it. You should assign oxidation numbers per element not just for the polyatomics as a whole. Let me give you a few examples of how this works.

• Consider CO₂. Oxygen is usually -2, and there are two of them, so that lone carbon must be +4, to sum up to 0, the overall charge on the molecule. C=+4, O=-2
• Consider P₂O₅. Oxygen is usually -2, and there are 5 of them, so the TWO phosphorus have to equal +10, so EACH phosphorus has an oxidation number of +5. P=+5, O=-2
• Consider H₂SO₄. Oxygen is usually -2, and hydrogen is almost always +1. That means that we have -8 for the oxygen and +2 for the hydrogen. That gives -6 total, meaning that the sulfur must be +6 to make the whole thing neutral. H=+1, S=+6, O=-2
• Consider the Cr₂O₇^-2 ion. In this case, our target number is the charge on the ion, which is -2, not 0. So, in this case we get Oxygen usually -2, and there are 7 of them, so that's a total of -14. Since the whole thing must equal -2, that means the two Chromiums TOGETHER must equal +12, so EACH chromium has to equal +6. Cr=+6, O=-2.

There are a few places where you may slip up. H is almost always +1, except in the case of metalic hydrides; in those cases (always of the formula MH_x, where M is a metal) H=-1. Also, there are a few exeptions to the O=-2 rule. If oxygen is bonded to Fluorine, such as in OF₂, fluorine being more electronegative will make the oxygen positive, so O=+2 in that case. Also, there are a few types of compounds like peroxides (O=-1) and superoxides (0=-1/2) where oxygen does not have a -2 oxidation number. These will be fairly rare, and you should only consider them where using O=-2 doesn't make sense, for example in H₂O₂, if O=-2 then H=+2, which makes no sense since H has only 1 proton. So in that case, O=-1 is the only way it works. However, these are rare exceptions, and I would expect almost ALL of the problems you will face in a first-year chemistry class to be more "Standard" types as I describe above.--Jayron32 06:09, 7 December 2009 (UTC)

Note that O being an oxidation state of (-1) is the reason why peroxides are such strong oxidants. Oxygen is more stable at oxygen state (-2), and so peroxides are susceptible to nucleophilic attack where one oxygen atom accepts electrons and pushes out hydroxide or alkoxide as the leaving group (cuz of the weakness of the oxygen-oxygen bond). John Riemann Soong (talk) 06:29, 7 December 2009 (UTC)

An important note is that "oxidation states sum to zero" ONLY in neutral compounds. If your compound is an an ion (for example, perchlorate or phosphate) or NAD+, then oxidation state will sum up to the charge of that ion. E.g. hydronium has an oxidation state +1. (It makes sense, right?) John Riemann Soong (talk) 06:33, 7 December 2009 (UTC)

This is helpful, thank you very much to all. Chrisbystereo (talk) 08:09, 7 December 2009 (UTC)

Value of a microchip

If you were to take all the metals and so on out of a chip (your choice, the newest Pentium, a digicam's image sensor, etc) and price them according to whatever tantalum/aluminum/titanium/cobalt/etc is going for, what would a chip's value be? I'm just curious what the difference between the cost of the components and the cost of the labor and such put into making it all work together. Has anyone ever even figured this out before? Dismas|^(talk) 04:46, 7 December 2009 (UTC)

The chip is basically a few grams of silicon, plastic, and maybe copper and iron. I can't imagine that the materials would be more than a few U.S. cents, if that much. The lion's share (99.99%) of cost of the chip itself is labor. --Jayron32 04:51, 7 December 2009 (UTC)

I'm very aware that the value would be small but I was just wondering how small. My job is to make them and I've been spending the last few weeks looking at them under a scope and this question popped into my head. Dismas|^(talk) 05:10, 7 December 2009 (UTC)

Well, you probably have more accurate measures on the amounts of metal deposited in your process; and if you discount everything that gets wasted when it's etched away, you probably end up with a chip that contains a few nanograms of aluminum, a few picograms of boron, and a couple milligrams of silicon. Other trace metals depend on your process. Perhaps a better way to price everything is to count the number of bottles of each chemical solution or metal ingots that you consume in a given day/week/whatever, and divide by the number of chips produced. Again, this doesn't account for waste material, so you have to do some estimation. Nimur (talk) 08:06, 7 December 2009 (UTC)

Pure silicon costs a lot more than impure. Does that difference count as labor to you? Metal ore in the ground is free for the taking. Making the base metal is all labor. Pretty much the cost of everything is just labor and energy. There is no "component price" for things, the only question is where do you draw the line and say "this is labor cost", and this is "component cost". I suppose - to you - it depends on if you buy it or make it. But globally there is no such line. To answer the question you are probably actually asking: I would suggest adding up the estimated total salary for everyone in your company, and subtract that from the gross income, and subtract profit. (If it's a public company you should be able to get those numbers.) Then you'll have things like overhead, and energy to include or not, as you choose. Ariel. (talk) 09:12, 7 December 2009 (UTC)

(either I was still sleepy, or I had an unnoticed ec - Ariel says essentially the same above) Also, the question is not very well-defined. For a microprocessor, you need very pure materials. A shovel of beach sand probably has most of the ingredients needed, but single-crystal silicon wafers are a lot more dear than that. If you pay bulk commodity price for standard-quality ingredients, the price of the material for a single chip is essentially zero. But in that case you will also need a lot of time and effort to purify them to the necessary level. --Stephan Schulz (talk) 09:13, 7 December 2009 (UTC)

Wouldnt a vast inclusion of cost be R&D? I remember someone quoting West Wing on this desk about pharmaceuticals that would be relevant: "The second pill costs 5 cents, its that first pill that costs 100 million dollars." Livewireo (talk) 18:18, 7 December 2009 (UTC)

Indeed. The cost is almost entirely R&D, I would think. That is a labour cost, though. --Tango (talk) 20:56, 7 December 2009 (UTC)

Nevermind. I said I make them, I didn't say I owned the company and had access to all the costs associated with making them. I just wanted to know how much it would be if I melted it down and sold the constituent metals and such. I didn't think I was being that unclear. I'll just assume it's vanishingly small. Dismas|^(talk) 20:19, 7 December 2009 (UTC)

It would cost far more to separate the components than the components would be worth. Your question is easy to understand, it just doesn't have an answer - not all questions do. --Tango (talk) 20:55, 7 December 2009 (UTC)

The fun of the question is how close to zero it is. Bus stop (talk) 21:10, 7 December 2009 (UTC)

Thank you, Bus stop. I think you get my question most of all. I didn't mention labor at all. Or R&D. Nor did I ever say anything about the cost of separating the components. Again, nevermind. Dismas|^(talk) 22:01, 7 December 2009 (UTC)

Ok, but you can't get round the purity issue mentioned above. There isn't a single value for silicon, say, it depends on the purity. How pure the silicon would be depends on how much labour you put into separating the components. --Tango (talk) 22:12, 7 December 2009 (UTC)

And the quantity of metals depends wildly on the actual die, photo masks, etc. As I mentioned above, you can estimate the masses of these constituent ingredients better than we can. Different mask patterns can leave as much as 100% or as little as 0% of a particular deposited layer - so there is no "in general" answer. You just have to estimate layer thickness and layer area for each stage of the process. Some typical numbers for areas and thicknesses might come out of articles like Self-aligned gate#Manufacturing process. Nimur (talk) 22:17, 7 December 2009 (UTC)

mesomeric versus inductive effects for the pka of catechol (ortho-diphenol)

I actually thought that o and p benzenediols should have higher pkas than phenol because of the destabilising mesomeric effect, but it seems that catechol (ortho-diol) has a pka of 9.5 (according to wikipedia). Google seems to say resorcinol (meta-diol) has a pka of 9.32 while para-diphenol is 9.8. This source seems to give a different frame of values.

My hypothesis is that the inductive effect is also at play, where having a carbanion resonance structure next to a (protonated) oxygen atom will stabilise it somewhat. And of course, the further away the two groups are from each other, the weaker the inductive effect, so it's why the para-diphenol would have the highest pka's of all the diphenols, while the meta-diol would barely see any mesomeric effect and mostly see the inductive effect. Is this reasonable? Is it supported by literature? John Riemann Soong (talk) 05:44, 7 December 2009 (UTC)

phenols as enols

I'm looking at this synthesis where a phenol is converted into a phenoxide and then used to perform a nucleophilic attack (in enol form) on an alkyl halide. My question is: why use lithium(0)? It seems a lot of trouble when you could just simply deprotonate phenol with a non-nucleophilic base like t-butyl hydroxide. Is it because a phenolate enolate is more nucleophilic at the oxygen? If so why not use something like lithium t-butyl hydroxide to bind the phenolate more tightly? John Riemann Soong (talk) 06:24, 7 December 2009 (UTC)

I disagree with "a lot of trouble". Weigh a piece (or measure a wire length) of metal, drop it in, and you're done. Seems no worse than measuring your strong base (often harder to handle and/or harder to measure accurately). And where does that base come from? Do you think it more likely to be a benefit or a problem to have an equivalent of t-butanol byproduct (the conjugate acid of your strong base) in the reaction mixture (note that the chosen solvent is non-Lewis-basic) and during product separation/purification? The answer to every one of your "why do they do it that way" is because "it was found empirically to work well enough and provide a good trade-off for results vs cost." Really. Again again, nothing "in reality" works as cleanly as on paper, so you really have to try lots of "seems like it should work" and you find that every reaction is different and it's very hard to predict or explain why a certain set of conditions or reactants is "best" (for whatever "best" means). It's interesting to discuss these, but I think you're going to get increasingly frustrated if you expect a clear "why this way?" answers for specific reactions. On paper, any non-nucleophilic base will always work exactly as a non-nucleophilic base, and that's the fact. In the lab, one always tries small-scale reactions with several routes before scaling up whatever looks most promising. DMacks (talk) 07:05, 7 December 2009 (UTC)

Along those lines, the best source for a certain reaction is the literature about that reaction. The ref you saw states "The use of lithium in toluene for the preparation of alkali metal phenoxides appears to be the most convenient and least expensive procedure. The procedure also has the merit of giving the salt as a finely divided powder." DMacks (talk) 07:12, 7 December 2009 (UTC)

Sorry I guess my experience with oxidation-state 0 group I and II metals so far has been with Grignard and organolithium reagents. From an undergrad POV, they are such an awful pain to work with (compared to titrating a base and acid-base extraction)! Also -- deprotonated phenols can act like enols? Why aren't aldol side reactions a problem to worry about during the synthesis of aspirin from salicylic acid? And why aren't enol ether side reactions a worry here? John Riemann Soong (talk) 07:25, 7 December 2009 (UTC)

The cited ref notes that the enol-ether side product is a huge problem (3:1 of that anisole product vs the "enolate α-alkylation" product they are primarily writing about). If the goal is "a difficult target", it doesn't matter if the reaction that gives it actually only gives it as a minor product compared to some other more likely reaction. The standard result of "phenoxide + SN2 alkylating agent" is O-alkylation, with other isomers being the byproduct. However, in general for enolates, the preference for O-alkylation vs C-alkylation is affected by solvent (especially its coordinating ability), electrophile, and metal counterion. It's unexpected to me that they get so much of it, but if there's any there and you want it badly enough, you go fishing through all the other stuff to get it. That's what makes this reaction worthy of publication...it does give significant amounts of this product and allows it to be purified easily from the rest. DMacks (talk) 09:16, 7 December 2009 (UTC)

phenol-type quinoline

What do you call a phenol-type quinoline with an hydroxyl group substituted in the 8-position? I'm trying to find out its pKa (in neutral, nonprotonated form), but it's hard to know without realising what its name is.

(Also, it is an amphoteric molecule right?) These two pkas appear to interact via resonance, making for some weird effects on a problem set ... (I'm considering comparative pH-dependent hydrolysis rates (intramolecular versus intermolecular) for an ester derivative of this molecule...) John Riemann Soong (talk) 07:30, 7 December 2009 (UTC)

Standard IUPAC nomenclature works pretty well for any known core structure: just add prefixes describing the location and identity of substituents. So quinoline with hydroxy on position 8 is 8-hydroxyquinoline (a term that gives about 132,000 google hits). Adding "pka" to the google search would help find that info. The protonation of these types of compounds is really interesting (both as structural interest and in the methods to study it)! All sorts of Lewis-base/chelation effects. DMacks (talk) 09:03, 7 December 2009 (UTC)

acidic proton question (Prilosec & Tagamet!)

Okay sorry for posting the 4th chem question in a row! I'm trying to figure out the acidic proton in two molecules, Tagamet and Prilosec. I'm given a pKa of 7.1 for the former and 4.0 and 8.8 for the latter. I don't know which ions the two in Prilosec the two pKas correspond to. (Possibly I feel there are much more basic and acidic sites, but they are not detailed or outside the range of discussion?)

With Tagamet imidazole is the the most obvious candidate for being a base with a pkB near 7, but I'm wondering why not the guanidine type residue? It has a nitrile group on it -- but how many pKa units would it raise? pKa of guanidine is 1.5, so plausibly a CN group could raise it to 7?

Oh yeah, and Prilosec. I'm ruling out the imidazole proton, but I feel that alpha-carbon next to the sulfoxide group is fairly acidic, cuz it has EWGs on both sides PLUS the carbanion could be sp2-hybridised if the lone pair helps "join" two conjugated systems. But the imidazole and pyridine lone pairs also look good for accounting for some of those pKas. Why aren't there 3 pKas? I think the imidazole-type motif in Prilosec is responsible for the pKa (pka of conjugate acid) of 8.8 -- but why the elevated pKa compared to normal imidazole? And why would the pKa of pyridine fall that low? (It has an electron donating oxygen substituted in para-position!) But assigning the pKas the other way round doesn't make sense either. Slightly disconcerted as I know these lone pairs are basic. John Riemann Soong (talk) 09:50, 7 December 2009 (UTC)

science

at firt there were judt two societies i.e hunting and gathering society but there was still life,the people were still living no equality was present evey one was equall and there was peace all over.But now a days cause of science there was no peace,no equality no respect every one is induldge in earning money.so wat should be hapened if the science and its invention are removed from our society? should we again start hunting and gathring society just 4 the sake of peace and equality? —Preceding unsigned comment added by Umair.buitms (talk • contribs) 13:31, 7 December 2009 (UTC)

What makes you think hunter-gatherer societies were peaceful? They generally had greater equality since there wouldn't be enough food to go around if there was an elite that didn't hunt or gather, but they certainly fought neighbouring tribes. Do you really want equality, though? Surely everyone having a low standard of living is worse than some people having a high standard of living and others having a higher standard, which is the case in the modern developed world. --Tango (talk) 13:38, 7 December 2009 (UTC)

I agree with Tango - there was unlikely to have been "equality" in the early days of humanity - and certainly no "peace". In modern times, there are still a few hunter-gatherer societies out there in places like the Amazon rainforest that science has not touched. For them, there is still warfare between tribes - women are still given one set of jobs and the men others - and there are still tribal leaders who rule the lower classes. The one place where equality is present is in "racial equality" - but that's only because they don't routinely meet other races because of the geography.

As for removing science and invention - our society literally could not exist that way. The idea that (say) 600 million Americans could just put on loincloths and start hunting and gathering is nuts! There would be nowhere near enough food out there for that to happen - without modern agriculture, we're completely incapable of feeding ourselves. We would need for perhaps 599 million people to die before the one million survivors could possibly have enough to eat.

I think your idyllic view of hunting & gathering is severely misplaced. It's a cruel, brutal existence compared to the relative peace and tranquility that is modern life.

SteveBaker (talk) 13:51, 7 December 2009 (UTC)

It a very common if very confused view, that all human problems are a product of modernity and so forth. It's true we have some new problems... but the problems of civilization are all there in part because living outside of civilization is so brutal. It is similar to the point of view that animals want to be "free"—most appear to want a stable food source more than anything else, because being "free" means starving half of the time. That's no real "freedom". --Mr.98 (talk) 14:44, 7 December 2009 (UTC)

At least Sabre Toothed Tigers are extinct this time around. APL (talk) 15:16, 7 December 2009 (UTC)

Do you have any references to support your utopian view of the hunter-gatherer societies? All evidence I've seen points to a society in which tribal warfare is common. Women are possessions. Children are expendable. And attempts to advance society are only accepted if they allow the tribe to attack the neighbors and steal more women and children. I feel that modern society is a bit more peaceful than that. -- kainaw™ 13:57, 7 December 2009 (UTC)

To be fair, women as possessions is more what you get after the arrival of basic agriculture (herding), when the link between sex and children is more clearly understood and the concept of 'owning' and inheriting is established. Societies everywhere have cared about their own children: they were not viewed as expendable, except in as much as 'people who are not my family/tribe' are viewed so. If children were really viewed as expendable, there wouldn't be any concern about continuation of the family and providing inheritance, and hence there would be no possessiveness of women: the whole 'women as possessions' thing is about ensuring the children they bear are verifiably the children of the man who thinks they're his: without that, there's no reason for the man to care if the woman has sex with other men. The OP may have a hopelessly utopian view, but I'm not convinced yours is any more accurate. If nothing else, the Old Testament gives us accessible sources written up to three thousand years ago: the overwhelming feeling I get from it is how little the way people think has changed in the most basic ways. It is full of people caring very much about their children, way back. 86.166.148.95 (talk) 18:50, 7 December 2009 (UTC)

I'm going to be all cheesey and link you to Billy Joels We didn't start the fire. 194.221.133.226 (talk) 14:06, 7 December 2009 (UTC)

I think even One Million Years B.C. was closer to the truth than the OP's utopian vision. Though their makeup probably wasn't as good :) Dmcq (talk) 14:27, 7 December 2009 (UTC)

OP, the viewpoint you expressed is known as anarcho-primitivism. You can read our wikipedia article, which includes views of both proponents and critics. See also Luddite, Neo-Luddism etc for less extreme versions of anti-modernism movements. Abecedare (talk) 15:39, 7 December 2009 (UTC)

Also, for what it's worth, the development of science and so-called modernity was really just the logical outcome of a successful hunter-gatherer society. It's a lot easier, efficient, and survivable to build a house and a farm instead of wandering around hoping you find food, water, and shelter. Agriculture leads to spare time, spare time leads to advancements, which lead to greater agriculture, which eventually leads to Twitter. You can't go back, nobody would chose death over relaxation and creativity. ~ Amory (u • t • c) 16:26, 7 December 2009 (UTC)

It's not that inevitable - plenty of societies didn't develop agriculture until they were introduced to it by other societies, some in modern times (eg. Australian Aborigines). Things wouldn't have needed to be too different for agriculture to have never been developed anywhere (or, at least, not developed until millennia later than it was). --Tango (talk) 16:34, 7 December 2009 (UTC)

These are basically value judgements we are all making. These are subjective answers we are giving. Not surprisingly we favor what we have. Bus stop (talk) 16:41, 7 December 2009 (UTC)

Re: not inevitable: I seem to recall that one of the ways archaeologists identify remains as early-domesticated goats rather than wild goats, is to look for signs of malnutrition. Captive goats were less well fed than wild goats. 86.166.148.95 (talk) 18:54, 7 December 2009 (UTC)

I've never heard that, but it makes some sense. Goats were often raised for milk, rather than meat, and they don't need to be particularly well nourished to produce milk (actual malnourishment would stop lactation - animals usually don't use scarce resources on their children if they are at risk themselves). --Tango (talk) 18:57, 7 December 2009 (UTC)

The actual experience of prehistoric hunter-gatherers is a serious bone of contention among anthropologists, made all the more difficult by various wild claims made by armchair researchers of the 18th and 19th centuries. (See state of nature and Nasty, brutish, and short). Among the complications are these: HGs lived in wildly diverse ecologies, meaning they had wildly diverse lifestyles, with wildly diverse advantages and disadvantages - how can you evaluate the lifestyle of an averaged out Inuit/San person meaningfully? Also, the few remaining HGs live at the very edges of the habitable earth, which makes it difficult to extrapolate what life was like in more normalized areas. In very, very, generic terms you can say this: people who lived the HG lifestyle worked a lot less per day than the farmers their descendants eventually became, they had few diseases compared to farmers, and they probably had more well-rounded diets than farmers. While there was surely enough sexual discrimination to go around, it was probably not nearly as bad as in the farming communities and the whole "slavery to acquisition" we in the modern world play to was pretty much non-existent; you can't build up wealth if you've got to slug everything on your back. On the other hand, they had relatively slow population expansion so when disasters did hit, it might spell the end to the band or tribe. Inter-band warfare was a real hit and miss kind of thing too - there were neighbours to be trusted and others that weren't, but with no central authority, there was really nobody "watching your back" if relations got out of hand. On the whole, it probably was quite a nice existence if you happened to be living in a reasonable area and didn't mind living your life within an animistic/mystical framework where you have enormous understanding of the surface of the world around you, but virtually no grasp of the real reason why anything happens. No books, no school, no apprenticeship, very little craft specialization beyond perhaps "women gather, men hunt" kind of thing. Matt Deres (talk) 21:43, 7 December 2009 (UTC)

Of course you can build wealth if you need to haul it around. Your form of wealth would most likely be draft animals so that you can carry more stuff around. Googlemeister (talk) 22:21, 7 December 2009 (UTC)

Domestication of animals is part of the road to civilisation. If we're talking about early hunter-gatherer societies (which I think we are - if we're talking about later h-g societies then you may be right), then they wouldn't have domestic animals. They wouldn't have had much to carry around. Simple clothes, stone tools, ceremonial items. Their economy was 99.9% food and I don't think they had the means to preserve it for long. --Tango (talk) 22:47, 7 December 2009 (UTC)

It does not need to be animals, slavery has been around for some time as well. Googlemeister (talk) 16:54, 8 December 2009 (UTC)

The basics of smoking meat have been known for a very long time, but I think it really came down to not wanting to carry the result around. In order to not exhaust an area, most HGs had to keep on moving at regular intervals and the use of pack animals was, while not completely unknown, not something widely employed. Dogs were probably in use for help with the hunt, but once you start semi-domesticating your prey animals you're not really hunting-gathering anymore - now you're herders, and eventually pastoralists, perhaps practising transhumance. Anthropologists use the term "pastoralist" in a more narrow sense than our article does, basically only using it for those groups that have only minimal physical goods and a high reliance on the herd animal. The classic example there are the Nuer. Matt Deres (talk) 01:33, 8 December 2009 (UTC)

Smoking meat might help you get through a bad winter, but it isn't going to allow you to build up a retirement fund. I think a HG would have two possible wealth levels - enough food for their tribe to survive and not enough food for their tribe to survive. I can't see any way they could have significantly more wealth than than having enough food to eat. --Tango (talk) 11:51, 8 December 2009 (UTC)

1. This subject would have been more appropriately placed on the Humanities Desk.
2. The OP is delusional.

Life, in the state of nature, is "solitary, poor, nasty, brutish, and short" leading to "the war of all against all."
  — Thomas Hobbes, Leviathan (1651)

B00P (talk) 23:24, 7 December 2009 (UTC)

Getting back to the OP: you may have been misled by the name into thinking there were two societies originally, one that hunted and one that gathered. In fact, hunter-gatherer is a generic name for pre-agricultural groups, almost all of which ate both types of foods: those (mostly animals) which some members (mostly male) had hunted, and those (mostly plants) which others (mostly female) had gathered. (Another name for these societies, should you wish to research further, is foragers.) It is true that most of these groups had to be mobile, to follow the food, and as such could carry little with them, so they did not accumulate wealth in the sense in which we understand it. However, there are always exceptions. One well-studied example are the Indigenous peoples of the Pacific Northwest Coast, who lived in a rich and fertile ecosystem, and particularly the Haida, who developed an impressive material culture -- so much so that they had to invent the potlatch in order to get rid of (or share around) the surplus. And that relates to another sort of wealth, a social and cultural wealth as opposed to a material one. Much harder to demonstrate than grave goods! BrainyBabe (talk) 23:29, 7 December 2009 (UTC)

@B00P- Please don't wp:BITE the newbies or post nonsense. Hobbes was a philosopher who never did any fieldwork, never studied the topic and just made shit up as he went along, pretending a state of nature had once existed so he could have an excuse to make up even more shit without any basis in reality. Explaining why you think such-and-such a policy is good is perfectly fine, inventing lies and making crap up out of whole cloth is the worst kind of academic fraud. You don't do yourself any favours by quoting him as if it was worth anything. Matt Deres (talk) 01:43, 8 December 2009 (UTC)

I think that in good times they had peace of mind beyond our wildest imagination. Bus stop (talk) 23:32, 7 December 2009 (UTC)

Indeed. So pervasive is the "solitary, poor, nasty, brutish, and short" lie that most people don't know that H-Gs in fact lived in familial groups, had no poverty, weren't particularly nasty or brutish, and lived longer, healthier lives than their farming cousins. I think that, if most people could see what kind of life they'd reasonably expect to lead, they'd choose hunting and gathering over farming any day of the week - less work, less disease, less worries, no boss - who wouldn't want that? Matt Deres (talk) 01:52, 8 December 2009 (UTC)

(edit conflict)I highly doubt the world would be able to support 6+ billion hunter gatherers. I would think that alone would answer the OP's question - if we, as a species, reverted back to hunting and gathering, it would require the deaths of billions. I would say that suggests that, "4 the sake of peace and equality", we definitely should not do this. TastyCakes (talk) 23:34, 7 December 2009 (UTC)

Well, death is the great equalizer ... so in some sense, "4 the sake of peace and equality", we should do this unless we do not value equality.... "I'll rant as well as thou." Nimur (talk) 18:09, 8 December 2009 (UTC)

Agriculture and settlements didn't necessarily make life better, but it made life more productive. Settled farmers can produce more food than can hunter-gatherers, and more food = more children.[1] Thinkquest estimates the Earth's carrying capacity for hunter-gatherers to be 100 million.[2], and the carrying capacity assuming the farming of all arable land as 30 billion people. As soon as human societies struck on the strategy of settling and farming this inevitably expanded either by cultural exchange or by the farmers progressively expanding their territory to the detriment of hunter-gatherers. We have pre-Western contact Australian Aboriginals and Bushmen/San as remaining examples of hunter-gatherers; Amazonian tribes aren't a great example as they derive from pre-Colombian civilizations who left behind the Terra preta, so they have traditions and social structures that persist from those times. Here's a study of hunter-gatherer societies at the end of the last ice age, which looks at societal transitions.[3] As for violence and peace, the death rate of adults of the Hiwi is about 2% per year, and the death rate in Cro-Magnon and our sibling species the Neanderthals is estimated to have been 6% per year.[4][5] Over half of deaths in the Aché pre-contact with Westerners were due to violence, so the idea that it is modern society, technology, science or civilization that causes violence is plain wrong, but it does make our killing more efficient and able to be done on a larger scale. Fences&Windows 18:42, 8 December 2009 (UTC)

Photon

Does the energy of a photon depend on the reference frame? I would think so, because observers in difference reference frames measuring the frequency of a photon will measure different values (because their clocks run at different rates), and E=hf. But then a parado seems to arise: If observer A measures the energy of a photon to be E, then an observer B moving relative to A should measure a lower energy, γE. But in B's reference frame, A should measure an energy of γA. So who measures what energy? —Preceding unsigned comment added by 173.179.59.66 (talk) 17:54, 7 December 2009 (UTC)

See redshift. Redshift is precisely photon energies being different in different frames. Redshift is determined by the relative velocity between the source and observer. The difference in velocity between two observers would mean each sees a different redshift - the one receding from the source faster (or approaching the source slower) will see the energy as lower. --Tango (talk) 18:04, 7 December 2009 (UTC)

There are other factors that influence the observed frequency besides the γ-factors. If everything is taken into account, there is no paradox. See doppler effect. Dauto (talk) 18:08, 7 December 2009 (UTC)

Also, see Relativistic Doppler effect, which extends the mathematics to apply to a wider range of relative velocities of reference frames, accounting for additional effects of relativity. Nimur (talk) 19:08, 7 December 2009 (UTC)

Okay, so basically you're saying that the equation relating the two observed frequencies would be the Doppler equation, regardless if the emitter is actually seen? —Preceding unsigned comment added by 173.179.59.66 (talk) 01:02, 8 December 2009 (UTC)

What do you mean by "regardless if the emitter is actually seen?" If you detect a photon than (by definition) the emitter is being seen. Dauto (talk) 09:00, 8 December 2009 (UTC)

What I meant was, you don't know the velocity of the emitter. But I realise now that it shouldn't matter. —Preceding unsigned comment added by 173.179.59.66 (talk) 11:29, 8 December 2009 (UTC)

And a related question (actually, this was the motivator for the first question): suppose that a photon strikes a proton (at rest in the lab frame) and produces a proton and a pion or something. The first question (this was an exam question) was to find the threshold energy, which I did without problem. The second question asked to find the momentum of the pion if the photon has the threshold energy. So my strategy was to find the velocity of the center of mass and then make that the velocity of the pion...how would you do this though? —Preceding unsigned comment added by 173.179.59.66 (talk) 01:17, 8 December 2009 (UTC)

If the photon has the threshold energy, and it actually has that interaction you describe, then all the energy is used up in creating the pion. There is no energy left to move anything, so pion and proton are stationary. But the photon had momentum so there will need to be some movement to carry that away. I guess you will need a simultaneous equation, one to preserve energy and one to preserve momentum. momentum of photon=momentum of proton+momentum of pion. (as vectors); energy of photon=kinetic energy of pion+kinetic energy of proton. Graeme Bartlett (talk) 03:16, 8 December 2009 (UTC)

Indeed, you need to solve the equations simultaneously. If you're clever, you'll be able to work out what directions the proton and pion fly off in. --Tango (talk) 15:59, 8 December 2009 (UTC)

It probably goes without saying, but the above equation for energy conservation should be:

energy of photon=kinetic energy of pion + energy equivalent of the pion's mass + kinetic energy of proton

Otherwise we're not accounting for the mass of the pion being created. TenOfAllTrades(talk) 16:22, 8 December 2009 (UTC)

Oops, missed that out. For the direction, I am guessing that the minimal energy situation will not have any side to side or up and down movement, so that only a a scalar needs to be considered. Photon hits proton and pion and proton head off in the same direction as the original photon. Your idea about changing the coordinates sounds wise. If you pick coordinates in which total momentum is zero, afterwards you will still have momentum zero. The minimal energy situation in this frame will be pion and proton sitting stationary at the same spot. Graeme Bartlett (talk) 20:57, 8 December 2009 (UTC)

The centre of mass frame does make things easier to calculate, but your final answer isn't very useful - yes, everything ends up at rest, but it's at rest in a frame that is moving very quickly compared to your lab. --Tango (talk) 22:54, 8 December 2009 (UTC)

It is usefull in so far as it stablishes that the proton and the pion will be moving together in the lab. ref. frame. Dauto (talk) 02:33, 9 December 2009 (UTC)

So at the minimum: energy of photon=hF=½v²(mass of pion+mass of proton) + energy equivalent of the pion's mass

Momentum of photon=hF/c=v(mass of pion+mass of proton)

Divide energy formula by momentum:

c=½v + (energy equivalent of the pion's mass)/(mass of pion+mass of proton)/v which you can solve for v perhaps (c=speed of light, h=Planck's constant F=frequency of photon). Graeme Bartlett (talk) 11:12, 9 December 2009 (UTC)

Area of Hong Kong

I was reading the question above about the size of California and I was wondering - has anyone ever gone and added up the total floor space in a dense city like Hong Kong, including all the floors in all those skyscrapers, as well as area on the ground, and compared that to its geographical area (1,104 square km, according to the article)? How much larger would Hong Kong, for instance, be? When viewed in that light, would the List of cities proper by population density change dramatically (ie cities with people living in big sky scrapers coming out looking better, ie less dense, than cities with lots of "1 story slums")? TastyCakes (talk) 19:23, 7 December 2009 (UTC)

I vaguely recall that such statistics (total habitable area) are commonly collected by governments, tax administration authorities, electric/water utilities, fire-departments, etc. I can't recall if "Total habitable area" is the correct name. I'm pretty sure that the statistic of habitable- or developed area (including multi-story buildings) as a ratio to total land area is commonly used for urban planning. Nimur (talk) 19:54, 7 December 2009 (UTC)

Floor Area Ratio. Sorry, the technical term had eluded me earlier. This article should point you toward more explanations of the usage of this statistic. Nimur (talk) 21:05, 7 December 2009 (UTC)

Ah ok, thanks for that. Have you ever heard of it being calculated for an entire city? TastyCakes (talk) 23:41, 7 December 2009 (UTC)

That's really the point - it's sort of a zoning ordinance metric for urban areas that's supposed to be more analytic than just capping the maximum number of stories per building. Apparently its use is widespread in urban Japanese zoning codes. It has also seen limited use in American urban planning, e.g. Boulder, Colorado. This source, Studio Basel (apparently a private architecture and urban studies insititute), claims that Hong Kong's floor area ratio is 10-12: "the highest urban density in the world". Nimur (talk) 00:32, 8 December 2009 (UTC)

(I thought I had posted this about six hours ago but I have just come back to find an edit conflict screen) Well the offiice space is 48 million square feet. I'll leave it to Steve Baker to add on something for the fractal cracks between the floorboards. This book has some information on residential space but its well out of date. SpinningSpark 00:23, 8 December 2009 (UTC)

Actually this is the argument for using floor area ratio as a density metric - it compares developed land area to actual habitable space by dividing the useful square footage by the area of its allocated plot - instead of dividing by some unknown estimate of the total city land area (which would include things like undeveloped hills, trees, spaces between buildings). FAR is more like an integral - it weights each building's floor space by the differential land-area unit that is allocated for it, and then accumulates and averages for the entire city. Cracks between floorboards aren't at issue - but unzoned land and undevelopable terrain are specifically not included in the total statistic. Nimur (talk) 00:37, 8 December 2009 (UTC)

Ah ok, I get that many square feet as being almost 4.5 square km, less than half a percent of Hong Kong's area. I can't imagine residential areas being hugely larger than that, but maybe I'm wrong? It seems that, even if residential space is several times office space, Hong Kong's usable surface area is only increased a few percent by all those sky scrapers and other buildings. Is that a fair assessment? TastyCakes (talk) 15:27, 8 December 2009 (UTC)

Only if you count "all land in the borders" as "usable." Another definition of "usable land" might be any land area that is zoned or districted for development. A floor area ratio of 10 means that you are multiplying the effective usable area by a factor of 10. Nimur (talk) 17:13, 8 December 2009 (UTC)

Converting from degrees K to F

Could someone please answer this question? Thanks, Kingturtle (talk) 19:58, 7 December 2009 (UTC)

The formula for converting K to F is F = 1.8K - 459.7 Googlemeister (talk) 20:02, 7 December 2009 (UTC)

That's rounded; F = 1.8K - 459.67 is the exact formula. "Degrees Kelvin" is obsolete terminology, by the way; they've been just called "kelvins" (symbol K, not °K) since 1968. For example, 273.15 K (kelvins) = 32°F (degrees Fahrenheit). --Anonymous, 21:04 UTC, December 7, 2009.

Google can actually answer these types of questions. What is 1.416785 × 10^32 kelvin in Fahrenheit? -Atmoz (talk) 21:54, 7 December 2009 (UTC)

WolframAlpha does this too, and gives other (scientific) information about the conversion for comparison. TastyCakes (talk) 23:37, 7 December 2009 (UTC)

Compact florescent bulbs

What is the acceptable temperature range at which you can use these lights? I ask because I want to know if I can use it outside when it is -50 deg, or if it will not work at that temperature. Googlemeister (talk) 19:59, 7 December 2009 (UTC)

Form our Compact fluorescent lamp article: CFLs not designed for outdoor use will not start in cold weather. CFLs are available with cold-weather ballasts, which may be rated to as low as -23°C (-10°F). (...) Cold cathode CFLs will start and perform in a wide range of temperatures due to their different design. Comet Tuttle (talk) 20:16, 7 December 2009 (UTC)

The packaging will indicate the acceptable range for the bulb. They are universally dimmer when cold, so this may be a persistent issue considering -50 (c or f) is 'pretty darn cold' in the realm of consumer products. —Preceding unsigned comment added by 66.195.232.121 (talk) 21:27, 7 December 2009 (UTC)

Inductive electricity through glass

With Christmas season here, I had an idea... Many wireless chargers use inductors to "transmit" electricity from a base unit to a device. Does anyone make that sort of thing that transmits electricity from inside the house to outside? I'm not considering a high-powered device. I'm considering the transmit/receive devices to be within an inch of each other on opposite sides of a window. -- kainaw™ 21:46, 7 December 2009 (UTC)

I think normal wireless rechargers should be able to transmit through glass. 74.105.223.182 (talk) 23:55, 7 December 2009 (UTC)

I figure it would, but I don't want to recharge my watch or toothbrush through a window. I'm looking for something to send electricity through a window. I would like to have two parts. The indoor part will have one end plug into an outlet and the other end stick to the inside of the glass. The other part will stick to the outside of the glass and have a socket to plug something into (like Christmas lights - which is what gave me the idea). -- kainaw™ 01:34, 8 December 2009 (UTC)

You'll need some conversion electronics to higher frequency. Inductive power transfer at 60 Hz requires very large coils. Most commercial devices I've seen operate at hundreds of megahertz and usually convert back to DC on the receiving-end. See Inductive charging, if you haven't already found that article. Nimur (talk) 07:14, 8 December 2009 (UTC)

It's a really good idea. If it's not expensive, you should try to bring it to market. (Although unless you can patent it, expect to be copied.) If you need more power, you can increase the area of the device. A suction cup will probably not work - they rarely can stay on for long. Ariel. (talk) 03:42, 8 December 2009 (UTC)